Spinal anaesthesia, also called spinal analgesia or sub-arachnoid block (SAB) , is a form of regional anaesthesia involving injection of a local anaesthetic into the Subarachnoid space, generally through a fine needle, usually 9 cm long (3.5 inches). For extremely obese patients, some anaesthesiologists prefer spinal needles which are 18 cm long (7 inches). The tip of the spinal needle has a point or small bevel. Recently pencil point needles have been made available (Whitacre, Sprotte, & others).
There are hyperbaric, isobaric, and hypobaric solutions of anaesthetics to choose for the spinal anaesthesia. Usually, the hyperbaric is chosen, as its spread can be effectively and predictably controlled by the anaesthesiologist, by tilting the patient.
Bupivacaine(Marcaine) is the local anaesthetic most commonly used, although lignocaine (lidocaine), tetracaine, procaine, ropivacaine, levobupivicaine and cinchocaine are also available. Sometimes a vasoconstrictor such as epinephrine is added to the local anaesthetic to prolong its duration. Of late, many anaesthesiologists are preferring to add opioids like morphine, fentanyl or buprenorphine, or non-opioids like clonidine, to the local anaesthetic used in spinal, to give a smoother 'effect' and to provide prolonged pain relief once the action of the 'spinal' has worn off.
Regardless of the anaesthetic agent (drug) used, the desired effect is to block the transmission of afferent nerve signals from peripheral nociceptors. Sensory signals from the site are blocked, thereby eliminating pain. The degree of neuronal blockade depends on the amount and concentration of local anaesthetic used, and the properties of the axon. Thin unmylenated C-fibres associated with pain are blocked first, while thick, heavily mylenated A-alpha motor neurons are blocked last. The desired result is total numbness of the area. A pressure sensation is permissible and often occurs due to incomplete blockade of the thicker A-beta mechanoreceptors. This allows surgical procedures to be performed with no painful sensation to the person undergoing the procedure.
Some sedation is sometimes provided to help the patient relax and pass the time during the procedure, but with a successful spinal anaesthetic the surgery can be performed with the patient wide awake. Spinal anaesthetics are limited to procedures involving most structures below the upper abdomen. To administer a spinal anaesthetic to higher levels may affect the ability to breathe by paralysing the intercostal respiratory muscles, or even the diaphragm in extreme cases (called a "high spinal", or a "total spinal", with which consciousness is lost), as well as the body's ability to control the heart rate via the cardiac accelerator fibres. Also, administration of spinal anaesthesia higher than the level of L1 can cause damage to the spinal cord, and is therefore usually not done.
Baricity refers to the density of a substance compared to the density of human cerebral spinal fluid. Baricity is used in anaesthesia to determine the manner in which a particular drug will spread in the intrathecal space. Hyperbaric solutions (for example, hyperbaric bupivacaine) are made heavy by adding dextrose to the mixture.